Predict the effect of decreasing the temperature on the amounts of reactants in the following reactions:

(a) ${\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{2}}\left(g\right)\mathbf{}\mathbf{+}\mathbf{}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{}\left(g\right)\mathbf{}\mathbf{\rightleftharpoons }{\mathbf{CH}}_{\mathbf{3}}\mathbf{}\mathbf{CHO}\mathbf{}\left(g\right)\mathbf{}\mathbf{}{\mathbf{\Delta H}}_{\mathbf{rxn}}^{\mathbf{0}}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{=}\mathbf{-}\mathbf{151}\mathbf{kj}$

(b) ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}\left(I\right)\mathbf{+}{\mathbf{O}}_{\mathbf{2}}\mathbf{}\left(g\right)\mathbf{}\mathbf{\rightleftharpoons }\mathbf{}{\mathbf{CH}}_{\mathbf{3}}{\mathbf{CO}}_{\mathbf{2}}\mathbf{H}\left(I\right)\mathbf{+}{\mathbf{HO}}_{\mathbf{2}}\mathbf{}\left(g\right)\mathbf{}\mathbf{}{\mathbf{\Delta H}}_{\mathbf{rxn}}^{\mathbf{0}}\mathbf{}\mathbf{=}\mathbf{-}\mathbf{451}\mathbf{kj}$

(c) $\mathbf{2}{\mathbf{c}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{4}}\mathbf{}\left(g\right)\mathbf{+}{\mathbf{O}}_{\mathbf{2}}\left(g\right)\mathbf{}\mathbf{\rightleftharpoons }\mathbf{2}{\mathbf{CH}}_{\mathbf{3}}\mathbf{CHO}\left(g\right)\mathbf{}\mathbf{}\left(\mathrm{exothermic}\right)$ (d)${\mathbf{N}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{4}}\mathbf{\rightleftharpoons }\mathbf{2}{\mathbf{NO}}_{\mathbf{2}}\mathbf{}\left(g\right)\mathbf{}\left(\mathrm{endothermic}\right)$

Reactions that involve the evolution of heat from the system are called exothermic reactions.

In these reactions, the energy of the products will be lower than the energy of the reactant, or the reaction involves the formation of a stable product. The excess energy in the reactant is released in the form of heat.

In reactions involving the formation of a higher energy product from a lower energy reactant, the extra energy is absorbed by the reactant.

These reactions are called endothermic reactions and involve the formation of unstable products.

If the forward reaction is exothermic in a reversible reaction, the backward reaction will be endothermic and vice versa. The effect of temperature is determined by the nature of the forward and backward reactions.

Generally, higher temperatures favor endothermic reactions, while exothermic reactions are favored at a lower temperatures. Therefore, as the temperature of a system increases, the equilibrium will shift to the endothermic side, and the rate of exothermic reaction increases with a decrease in temperature.

(a)${\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{}\mathbf{+}\mathbf{}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{}\mathbf{\rightleftharpoons }{\mathbf{CH}}_{\mathbf{3}}\mathbf{}\mathbf{CHO}\mathbf{}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{}\mathbf{}{\mathbf{\Delta H}}_{\mathbf{rxn}}^{\mathbf{0}}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{=}\mathbf{-}\mathbf{151}\mathbf{kj}$

In the given reaction, the enthalpy of the reaction has a negative sign. This means that heat is liberated during the reaction and the forward reaction is exothermic.

As temperature decreases, the rate of the exothermic reaction increases. Therefore, the rate of forward reaction increases, and more reactants will be converted into products.

Therefore, a decrease in temperature causes a decrease in reactant concentration.

(b) ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}\mathbf{\left(}\mathbf{I}\mathbf{\right)}\mathbf{+}{\mathbf{O}}_{\mathbf{2}}\mathbf{}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{}\mathbf{\rightleftharpoons }\mathbf{}{\mathbf{CH}}_{\mathbf{3}}{\mathbf{CO}}_{\mathbf{2}}\mathbf{H}\mathbf{\left(}\mathbf{I}\mathbf{\right)}\mathbf{+}{\mathbf{HO}}_{\mathbf{2}}\mathbf{}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{}\mathbf{}{\mathbf{\Delta H}}_{\mathbf{rxn}}^{\mathbf{0}}\mathbf{}\mathbf{=}\mathbf{-}\mathbf{451}\mathbf{kj}$

The forward reaction of the given reaction is exothermic as the enthalpy has a negative sign. Therefore, a decrease in temperature increases the rate of the forward reaction.

Then, more reactants will be converted into products. Therefore, a decrease in temperature causes a decrease in reactant concentration.

(c)$\mathbf{2}{\mathbf{c}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{4}}\mathbf{}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\mathbf{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{}\mathbf{\rightleftharpoons }\mathbf{2}{\mathbf{CH}}_{\mathbf{3}}\mathbf{CHO}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{}\mathbf{}\mathbf{\left(}\mathbf{exothermic}\mathbf{\right)}$

The forward reaction is given as exothermic. Therefore, a decrease in temperature increases the rate of the forward reaction.

More reactants will be converted to products, and a decrease in temperature will cause a decrease in reactant concentration.

(d) ${\mathbf{N}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{4}}\mathbf{\rightleftharpoons }\mathbf{2}{\mathbf{NO}}_{\mathbf{2}}\mathbf{}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{}\mathbf{\left(}\mathbf{endothermic}\mathbf{\right)}$

The forward reaction is endothermic. Therefore, a decrease in temperature will decrease the rate of the forward reaction.

The backward reaction will be exothermic, and therefore, its rate will increase with a decrease in temperature. More products will be converted into reactants.

Therefore, a decrease in temperature will cause an increase in reactant concentration.